This invention relates generally to optical communications systems. More particularly, it provides a novel class of optical interleavers with substantially minimized dispersion for multiplexing or de-multiplexing of optical signals.
Wavelength division multiplexing (WDM) has emerged as the standard technique to transmit information in fiber-optic networks. This is because as the bandwidth of fiber data increases, electronic sorting becomes increasingly complex, while wavelength routing becomes ever more practical and elegant.
In a WDM system, each optical fiber simultaneously carries many different communications channels in light of respectively different wavelengths. Each channel is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
WDM systems use components generically referred to as optical interleavers to combine, split, or route optical signals of different channels. Interleavers typically fall into one of three categories, multiplexers, de-multiplexers and routers. A multiplexer takes optical signals of different channels from two or more different input ports and combines them so that they may be coupled to an output port for transmission over a single optical fiber. A de-multiplexer performs the opposite process, that is, it decomposes an optical signal containing two or more different channels according to their wavelength ranges and directs each channel to a different dedicated fiber. A router works much the same way as a de-multiplexer; however, a router can selectively direct each channel according to control signals to a desired coupling between an input channel and an output port.
FIG. 1 depicts a typical optical interleaver 999 of the prior art as described in U.S. Pat. No. 5,694,233, which is incorporated herein by reference. A WDM signal 500 containing two different spectral sets 501, 502 enters interleaver 999 at an input port 11. AS used herein, the term xe2x80x9cspectral setxe2x80x9d refers to a particular range of wavelengths or frequencies that defines a unique information signal. A first birefringent element 30 spatially separates WDM signal 500 into horizontal and vertically polarized components 101 and 102 by a horizontal walk-off. Component signals 101 and 102 both carry the full frequency spectrum of the WDM signal 500.
Components 101 and 102 are coupled to a polarization rotator 40. The rotator 40 selectively rotates the polarization state of either signal 101 or 102 by a predefined amount. By way of example, in FIG. 1 signal 102 is rotated by 90xc2x0 so that signals 103, 104 exiting rotator 40 are both horizontally polarized when they enter a wavelength filter 61.
Wavelength filter 61 selectively rotates the polarization of wavelengths in either the first or second spectral set to produce filtered signals 105 and 106. For example, wavelength filter 61 rotates wavelengths in the first spectral set 501 by 90xc2x0 but does not rotate wavelengths in the second spectral set 502 at all.
The filtered signals 105 and 106 enter a second birefringent element 50 that vertically walks off the first spectral set into beams 107, 108. The second spectral set forms beams 109, 110.
A second wavelength filter 62 then selectively rotates the polarizations of signals 107 and 108, but not signals 109 and 110, thereby producing signals 111, 112, 113, 114 that have polarizations parallel to each other. A second polarization rotator 41 then rotates the polarizations of signals 111 and 113, but not the polarizations of signals 112 and 114. The resulting signals 115, 116, 117, and 118 then enter a third birefringent element 70. Note that second wavelength filter 62 may alternatively be replaced by a polarization rotator suitably configured to rotate the polarizations of signals 111 and 113, but not 112 and 114.
Third birefringent element 70 combines signals 115 and 116, into the first spectral channel, which is coupled to output port 14. Birefringent element 70 also combines signals 117 and 118 into the second spectral channel, which is coupled into output port 13.
As described above, interleaver 999 operates as a de-multiplexer. By operating interleaver 999 in reverse, i.e., starting with spectral sets 501, 502 at ports 13 and 14 respectively, interleaver 999 operates as a multiplexer. Furthermore, by suitably controlling the polarization rotation induced by rotators 40 and 41, interleaver 999 may be configured to operate as a router.
Interleaver 999 described above advantageously uses wavelength filters to separate an input WDM optical signal containing two spectral sets by way of different polarization modes and subsequently exploits the birefrigent walk-off effect to spatially separate different polarization modes, thereby de-multiplexing the input WDM optical signal. The use of the wavelength filters and birefrigent materials, however, inadvertently introduces various dispersion effects, which would degrade the performance of fiber-optic networks if uncompensated for. For instance, there is Polarization Mode Dispersion (PMD) known in the art, owing to the fact that different polarization modes traverse different optical path lengths in a birefrigent material. Moreover, since a wavelength filter is typically composed of a stacked plurality of birefrigent waveplates, different wavelengths of light undertake different polarizations in various constituent waveplates of a wavelength filter; and different polarizations subsequently lead to different optical path lengths. Hence, there is also Wavelength-Filter-Induced-Dispersion (WFID) that is both chromatic and polarization-related. Therefore, care must be taken to ensure that various dispersion effects are substantially minimized in an optical interleaver.
As fiber-optic systems rapidly spread as the backbone of modern communications networks, there is a need for optical interleavers in which dispersion effects are properly accounted for. The desired optical interleavers should also have a simple and low-cost assembly.
Accordingly it is a principal object of the present invention to provide a line of optical interleavers in which a novel beam-swapping element is utilized. Moreover, efforts are painstakingly made in the optical interleavers of the present invention to minimize various dispersion effects. It is a further object of the present invention to provide methods for constructing these novel optical interleavers.
An advantage of the beam-swapping element of the present invention is that it provides an effective and inexpensive alternative to the second polarization rotator and wavelength filter employed in the prior art optical interleaver as shown in FIG. 1, hence rendering a simple and low-cost assembly to an optical interleaver of the present invention. The use of the beam-swapping element further avoids undesirable complications such as dispersion effects. Another significant advantage of the optical interleavers of the present invention is that they present the first kind in the art in which various dispersion effects are substantially minimized. Such characteristics are highly desirable in fiber-optic networks.
These and other objects and advantages will become apparent from the following description and accompanying drawings.
The present invention provides an optical interleaver comprising a first birefringent element that decomposes and spatially separates an input WDM signal carrying first and second spectral sets into first and second beams with orthogonal polarizations. The first and second spectral sets are substantially complementary. A first wavelength filter, optically coupled to receive the first and second beams, decomposes the first beam into third and fourth beams and the second beam into fifth and sixth beams, by preferentially rotating the polarization of the second (or the first) spectral set in each of the first and second beams by 90-degree. Upon emerging from the first wavelength filter, the third and fifth beams carry the first spectral set with orthogonal polarizations, and the fourth and sixth beams carry the second spectral set with orthogonal polarizations. A second birefringent element, optically coupled to the first wavelength filter, spatially separates the four beams by way of the birefrigent walk-off effect. Upon emerging from the second birefrigent element the four beams are spatially positioned such that they can be construed as travelling along the four corners of an imaginary xe2x80x9crectangular propagation pipexe2x80x9d, with the third and fifth beams carrying the first spectral set diagonally opposing each other, and the fourth and sixth beams carrying the second spectral set diagonally opposing each other. A beam-swapping element is optically coupled to receive the third and sixth beams, or the fourth and fifth beams, from the second birefrigent element. Upon passing through the beam-swapping element, the third and fifth beams become positioned such that they can be construed as falling on a first side-plane of the imaginary xe2x80x9crectangular propagation pipexe2x80x9d described above, and the fourth and sixth beams become positioned such that they can be construed as falling on a second side-plane of the imaginary xe2x80x9crectangular propagation pipexe2x80x9d, where the first and second side-planes are parallel to each other. The third and fifth beams are then combined into a first output signal carrying the first spectral set, and the fourth and sixth beams are combined into a second output signal carrying the second spectral set, by way of a third birefrigent element. The two output signals may be further directed to two output ports.
The beam-swapping element in the present invention can be in the form of a hexagon plate, or parallelogram plate, comprising first and second faces parallel to third and fourth faces respectively. The four faces are oriented such that when two parallel beams, e.g., the third and sixth beams (or the fourth and fifth beams) in the above embodiment, are incident on the first and second faces, they emerge from the third and fourth faces respectively, thereby xe2x80x9cswappingxe2x80x9d in position. The beam-swapping element can also be a Dove prism known in the art of optics, where two slanted, non-parallel faces are utilized. As such, when two parallel beams (e.g., the third and fifth beams in the above embodiment) are incident on the first slanted face of a Dove prism, they emerge from the second slanted face in such a way that the two beams remain parallel, however xe2x80x9cswappedxe2x80x9d in position.
The optical interleaver of the present invention further comprises a compensation assembly, for ensuring that upon being combined various dispersion effects in each and every beam have been substantially minimized. The compensation assembly utilizes various arrangements of optical elements to substantially equalize the optical path lengths of the beams upon being combined. The compensation assembly further advantageously exploits the use of a second wavelength filter to cancel out the dispersion effects the first wavelength filter has inflicted on the beams.
As such, the optical interleaver of the present invention constitutes the first kind in the art in which various dispersion effects are substantially minimized. These dispersion-minimized optical interleavers would be highly desirable in fiber-optic networks. A further advantage of the optical interleavers of the present invention is that routing is accomplished while conserving substantially all optical energy available in the input WDM signal. That is, both the horizontal and vertical polarized components are used and recombined to provide the output signals, resulting very few loss through the optical interleaver.
The optical interleaver of the present invention can be configured to operate as a multiplexer, a de-multiplexer, or a router, as depicted in the drawings and the detailed description that follow.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.